1. Field of the Invention
This invention relates to a radiation image forming method and apparatus for obtaining at least either one of a volume signal and a tomographic image signal representing an image of an object. This invention particularly relates to a radiation image forming method and apparatus, wherein radiation image signals representing radiation images of an object are detected via scattered radiation removing means, which removes radiation scattered by the object, and at least either one of a volume signal and a tomographic image signal representing an image of an object, is obtained from the radiation image signals.
2. Description of the Prior Art
In the fields of medical images, systems for detecting two-dimensional radiation image signals (e.g., tomographic image signals), such as computed tomography scanners (CT scanners) and magnetic resonance imaging systems (MRI systems), have heretofore been used widely. (Such systems will hereinbelow be referred to as the radiation image detecting systems for two-dimensional images.)
Also, recently, research has been conducted to detect three-dimensional radiation image signals. As techniques for detecting three-dimensional radiation image signals, for example, helical CT and cone-beam CT have been proposed. (Such techniques are described in, for example, "Present State and Future of Cone-Beam CT Development," Image Information (M), pp. 122-127, January 1988; and Japanese Unexamined Patent Publication No. 9(1997)-253079.)
With the cone-beam CT, a radiation source and a two-dimensional radiation detector are rotated around an object, radiation is irradiated from the radiation source to the object, and a three-dimensional radiation image signal (i.e., a volume signal) representing the object image is acquired from radiation image signals (projected image signals), which have been detected at respective positions of rotation by the radiation detector. The systems for detecting three-dimensional radiation image signals will hereinbelow be referred to as the radiation image detecting systems for three-dimensional images. Also, the aforesaid radiation image detecting systems for two-dimensional images and the radiation image detecting systems for three-dimensional images will hereinbelow be referred to simply as the radiation image detecting systems.
In the radiation image detecting systems described above, such that radiation scattered by an object may not be detected, a grid comprising a material impermeable to radiation, such as lead, and a material permeable to radiation, such as aluminum or wood, which are arrayed alternately at a small pitch of, e.g., 4.0 pieces/mm, is often located between the object and radiation detecting means, and image recording operation is performed in this state. The utilization of the grid is advantageous in that radiation scattered by the object does not impinge upon the radiation detecting means, and therefore radiation image signals with high contrast can be obtained.
However, in cases where an image recording operation is performed by utilizing the grid, a stripe-like grid pattern is detected together with the object image by the radiation detecting means. In the radiation image detecting systems described above, the volume signal or the tomographic image signal is obtained from the radiation image signals, which have been detected by the radiation detecting means. Therefore, in such cases, the volume signal or the tomographic image signal is obtained from the radiation image signals carrying the information of the object image and the grid pattern. As a result, an artifact due to the grid pattern occurs in the reconstructed three-dimensional image or the reconstructed tomographic image. Accordingly, the problems occur in that a correct image cannot be reconstructed, and an image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, cannot be obtained.